Power transmission lines are a crucial part of any electrical distribution system, as they provide a path to transfer power between generation and consumer. Power transmission lines are closely interconnected for a reliable operation. Economic and environmental requirements lead to operation of power transmission lines close to their operating limits. Any fault will cascade into a system wide disturbance causing widespread outages for a tightly interconnected system operating close to its limits. Therefore, transmission protection systems are set up to identify faults. The main task for the transmission line protection lies in reliably detecting faults compromising the security of the system.
For differential protection systems of power transmission lines, protection devices at both ends of the transmission line measure a representation of a power line quantity such as current and compare the measured quantity of both ends. Under normal fault-free conditions, the quantity is the same at both ends. Deviating measurements indicate a fault on the transmission line and trigger protection actions such as opening a circuit breaker. A prerequisite for the laid out differential protection system is precise synchronization of the measurement devices on both ends.
For synchronizing, distributed protection device clocks are aligned to a master clock of some sort. In order to do so, a common clock source sends a synchronization signal for example, through a communication network. Each clock of the distributed protection devices receives the synchronization signal and performs necessary adjustments accordingly. Generally synchronization signals are sent from a global positioning system (GPS) satellite clock via direct or indirect satellite communication, or from a network master clock via a communication network to which the protection devices are connected.
However, synchronization with a master clock can neither be always reliable nor cost effective. Reception of a GPS based synchronization signal can be costly and even unreliable, as communication with the satellite can be interrupted for example, by radio interference or severe weather conditions impairing signal propagation. In addition, network-based synchronization assumes communication delays between clocks to be symmetric, i.e. equal in both directions, an assumption which is not generally guaranteed.